The relative speed of longitudinal sound in clathrate hydrates and in ice has been estimated to an accuracy of about 1/2%. It is 0.939 and 0.945 for the full structure I and structure II clathrates, respectively, and 1.011 for the empty clathrate of both kinds. The difference of the speed of sound in a porous rock when filled with ice and with clathrate is about 240ϕ(ur/ui)3 m s-1, where ϕ is the volume fraction of the ice or clathrate and ur and ui are the speeds of sound in rock containing ice or clathrate, and in ice, respectively.